Monte Carlo calculation of volumetric modulated arc therapy and helical tomotherapy dose ...

178 Pages·2013·8.41 MB·English

Monte Carlo calculation of volumetric modulated arc therapy
and helical tomotherapy dose distributions for stereotactic
ablative radiotherapy lung treatments
by
Jason Belec
B.Sc. (University of Ottawa) 2002
M.Sc. (McGill University) 2004
A thesis submitted to the
Faculty of Graduate and Postdoctoral Affairs
in partial fulfillment of the requirements
for the degree of
Doctor of Philosophy
in
Physics
Ottawa-Carleton Institute for Physics
Department of Physics, Carleton University
Ottawa, Ontario
Copyright © 2012, Jason Belec
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Abstract
The aim of this study was to calculate realistic dose distributions that include the
continuous deformation of organs and motion of treatment units using four dimensional
Monte Carlo methods for both volumetric modulated arc therapy and helical tomotherapy
stereotactic ablative radiotherapy lung treatments.
We present a method to perform position-probability-sampled Monte Carlo dose
calculations in the BEAMnrc and DOSXZYnrc user codes of EGSnrc. The method
includes full accelerator head simulation of conventional and helical tomotherapy
treatment units and a realistic representation of machine continuous motion via the
sampling of a time variable. The method simplifies the simulation process, improves the
dose calculation accuracy and involves an acceptably small change in computation time.
Absolute dose agreement for static fields between Monte Carlo calculations and
measurements is within 2 % /1 mm. Absolute dose agreement between Monte Carlo and
treatment planning system for four types of treatment techniques (3D-conformal
radiotherapy, step-shoot intensity modulation radiotherapy, helical tomotherapy and
volumetric modulated arc therapy) was determined to be within 3 % / 3 mm.
We also present a method to modify the DOSXYZnrc user code to account for the
continuous intra-fraction deformation of the patient geometry. We implemented two
methods in the user code (density interpolation, density mapping) to update the patient
geometry (transport grid densities) as a function of time and two methods (voxel average,
voxel center) to map back the energy deposited in the time dependant transport grid to a
reference grid. We also provide examples of mathematical phantom tests and
experimental measurements used to validate the implementation of these methods.
Finally, we present an example of the application of the method for stereotactic
ablative radiotherapy lung treatments with intra-fraction motion. The results show that
breathing motion is appropriately addressed with the internal target volume method for
the cases studied. For both helical tomotherapy and volumetric modulated arc therapy
stereotactic ablative radiotherapy lung treatments, the impact of interplay effects for
prescription doses of 54 Gy or 60 Gy resulted in a change of the target dose volume
histogram of less than 2 Gy scored at 10 % and 90% of the volume.
Acknowledgements
The research for this thesis was performed under the auspices of the Carleton University
Physics Department and the Ottawa Hospital Cancer Center Medical Physics
Department. I would like to express my gratitude to my supervisor, Dr Brenda Clark, for
her continued guidance, support and encouragement throughout this project. I would also
like to thank several Carleton University physics professors (Dr Paul Johns, Dr Gerald
Oakham and Dr Manuella Vincter) for teaching me about physics and striving to get the
best out of their students.
I would like to thank Blake Walters and Ali Elsayed for very helpful discussion
regarding the BEAMnrc and DOSXYZnrc user codes, Jason Smale and Dr Jason
Pantarotto for useful discussion, Dr Dave Rogers for allowing access to the physics
computation cluster and G. Wilson for supplying the iCom listening software. I also
acknowledge Elekta AB for funding this work and supplying technical details used in the
accelerator modeling. I would also like to thank the entire medical physics staff at the
Ottawa Hospital Cancer Center who have always been eager to help. In particular, I am
grateful to Dr Nicolas Ploquin and Dr Dan La Russa whose assistance was important.
Last, but not least, I must thank my parents, Suzanne and Yvon, for their
continuous encouragements and support. I also want to thank the love of my life, Isabelle,
for all the support she has given me over the past few years. Finally, I also thank my three
beautiful children (Loic, Anne-Sophie and Elliot) for the long sleepless nights.
Statement of originality
This thesis describes the results of the author's research conducted primarily at the
Ottawa Hospital Cancer Centre during the course of the Ph.D. program of Carleton
University. Most of these results have been published or submitted in either the Medical
Physics or Physics in Medicine and Biology journals, and have been presented at both
national and international conferences. The details of where the results of these
publications may be found in the thesis, together with the author's contributions, are
provided below:
PAPER I:
J. Belec, N. Ploquin, D. J. La Russa, B. G. Clark: Position-probability-sampled
Monte Carlo calculation of VMAT, 3DCRT, step-shoot IMRT, and helical tomotherapy
dose distributions using BEAMnrc/DOSXYZnrc Med. Phys. 38, 948 (2011)
The results of this paper constitute a part of Chapter 2 which discusses Monte Carlo
modeling of accelerator head and external beam radiation treatment delivery. The author
performed all of the work, prepared the manuscript for publication, and made the
necessary revisions following the review process, all under the supervision of Dr. Brenda
Clark. The results from this paper were also presented by the author at the 2010 COMP
conference (oral presentation). Dr N. Ploquin provided assistance with the use of the
Monaco treatment planning system and Dr D. J. La Russa provided assistance with the
compilation of the BEAMnrc / DOSXYZnrc shared library mode.
PAPER II:
(submitted) J. Belec, B. G. Clark: Monte Carlo calculation of VMAT and helical
tomotherapy dose distributions for lung stereotactic treatments with intra-fraction motion
Phys. Med. Biol. (2012)
The results of this paper constitute a part of Chapter 3 and Chapter 4 which discusses
Monte Carlo modeling of patient deformation. The author performed all of the
measurements and prepared the manuscript for publication under the supervision of Dr.
Brenda Clark. The results from this paper were also presented by the author at the 2011
AAPM conference (oral presentation).
Table of Contents
Abstract...................................................................................................................................ii
Acknowledgements...............................................................................................................iv
Statement of originality..........................................................................................................v
Table of Contents.................................................................................................................vii
List of Tables.........................................................................................................................ix
List of figures..........................................................................................................................x
Abbreviations....................................................................................................................xviii
1 Background......................................................................................................................1
1.1 Introduction..............................................................................................................1
1.2 Radiation therapy....................................................................................................1
1.3 Treatment delivery..................................................................................................2
1.3.1 Conventional treatment unit...........................................................................2
1.3.2 Helical tomotherapy treatment unit...............................................................4
1.3.3 Robotic treatment unit....................................................................................7
1.4 Dose distributions...................................................................................................9
1.4.1 Representation of dose distributions..............................................................9
1.4.2 Measurements of dose distributions.............................................................12
1.4.3 Calculations of dose distributions................................................................25
1.5 Stereotactic ablative radiotherapy (SABR) lung treatment................................34
1.6 Patient motion.......................................................................................................38
1.6.1 Setup motion.................................................................................................38
1.6.2 Image registration.........................................................................................39
1.6.3 Respiratory motion.......................................................................................43
1.7 Thesis outline........................................................................................................49
2 Treatment unit modeling...............................................................................................52
2.1 Introduction...........................................................................................................52
2.2 Materials and method............................................................................................56
2.2.1 Modifications to BEAMnrc/DOSXYZnrc..................................................56
2.2.2 User interface................................................................................................59
2.2.3 Monte Carlo model of a conventional treatment unit.................................64
2.2.4 Monte Carlo model of a HT treatment unit................................................68
2.2.5 Measurements for validating the Monte Carlo models...............................73
2.2.6 Treatment planning systems........................................................................74
2.3 Results....................................................................................................................75
2.3.1 Modifications to BEAMnrc/DOSXYZnrc..................................................75
2.3.2 Validation of the MC model of a conventional treatment unit...................77
2.3.3 Validation of the MC model of a HT treatment unit..................................88
2.3.4 Testing the MC model of a conventional treatment unit............................93
2.3.5 Testing the MC model of a HT treatment unit............................................97
2.4 Discussion............................................................................................................100
2.5 Conclusion...........................................................................................................104
3 Patient modeling..........................................................................................................106
3.1 Introduction.........................................................................................................106
3.2 Materials and methods........................................................................................106
3.2.1 Patient geometry........................................................................................106
3.2.2 Registration of energy deposition...............................................................115
3.2.3 Validation....................................................................................................121
3.3 Results................................................... 127
3.4 Discussion...........................................................................................................128
3.5 Conclusion..........................................................................................................132
4 SABR lung treatment planning study.........................................................................134
4.1 Introduction.........................................................................................................134
4.2 Materials and method..........................................................................................135
4.3 Results and Discussion.......................................................................................136
4.4 Discussion and conclusion.................................................................................140
5 Summary......................................................................................................................149
6 References...................................................................................................................152
viii
List of Tables
Table 1 Effective atomic number Zeff, number of electrons per gram pe and mass density
pm of the phantom materials (Kron, 1999)...........................................................................24
Table 2 Overview of patient characteristics used to study the impact of intra-fraction
motion in SABR lung treatments. The tumour-isocentre distance is specific to helical
tomotherapy and represents the distance between the machine isocenter (center of a
transverse CT image) and the location of the tumor center (GTV centroid). A larger
tumor-isocentre distance increase the impact of gantry angle discretization as explained
in figure 2.11. (Source: Paper II)......................................................................................137